The test never actually defined a function named 'alias1', not even
when zsh 5.4 warned it about this (see 9523d6d49c,
which was wrong and is hereby reverted).
The test that's now failing has been filed as issue #558.
zsh-syntax-highlighting / highlighters
Navigate into the individual highlighters' documentation to see
what styles ($ZSH_HIGHLIGHT_STYLES keys) each highlighter defines.
Refer to the documentation on highlighters for further information.